1. Technical Field of the Invention
The present invention pertains to method and apparatus for sum frequency generation in optical fibers and, in particular, to self-organized, phase-matched second harmonic generation in optical fibers.
2. Description of the Prior Art
Much activity has taken place recently in the field of second-harmonic generation in optical fibers in which a photo-induced effect forms a dipole-allowed, second-order susceptibility which is phasematched by the alternation of the sign of the nonlinearity along an optical fiber. For example, such effects have been reported in a paper entitled "Self-Organized Phase-Matched Harmonic Generation in Optical Fibers" by R. H. Stolen and H. W. K. Tom, Optics Letters, Vol. 12, No. 8, August, 1987, pp. 585; a paper entitled "Dye Laser Pumped by Nd:YAG Laser Pulses Frequency Doubled In An Optical Fiber," by U. Osterberg and W. Margulis, Optics Letters, Vol. 11, 1986, pp. 516; and a paper entitled "Experimental Studies On Efficient Frequency Doubling In Glass Optical Fibers," Optics Letters, Vol. 12, 1987, pp. 57. These papers disclose that second harmonic generation may be achieved in germania-phosphorus doped silica fibers after several hours of illumination from a Q-switched, mode-locked Nd:YAG laser having pulses with peak intensities of radiation of 100-200 GW/cm.sup.2. In effect, the fiber became a self-organized second harmonic generator.
The paper by Stolen and Tom discloses that the production of, or the conditioning of, the fiber to produce second harmonic radiation could be shortened substantially from many hours to several minutes by simultaneously illuminating the fiber with radiation at 0.532 um and radiation at 1.064 um. In fact, Stolen and Tom report that they produced the second-harmonic nonlinearity in a fiber in about five minutes as compared to the tens of hours required without the "seeding" provided by the 0.532 um harmonic radiation.
The above-cited papers disclose that the authors achieved the self-organized second harmonic generation in a single-mode germanium doped fused silica fiber having a 0.2 wt % co-doping of phosphorus. In fact, the above-cited papers consider the phosphorus to be essential to the production of the second harmonic generation. Additionally, they consider that the only effect the germanium doping has is to appropriately modify the index of refraction of the core to a value which is necessary to develop an appropriate NA (numerical aperture) for introducing radiation into the single-mode core.
However, several recent papers have disclosed that self-organized second harmonic generation can be obtained in germanium doped fibers. Such disclosures occurred in a paper entitled "Second-Harmonic Generation in Ge-Doped Fibers With a Mode-Locked Kr.sup.+ Laser" by B. Valk, E. M. Kim and M. M. Salour Appl. Phys. Let., Vol. 51, No. 10, Sept. 7, 1987, p. 722 and an abstract entitled "Second-Harmonic Generation in Single-Mode and Multimode Fibers" by M. A. Saifi and M. J. Andrejco, Paper No. ThD5, OFC/OFS 88 Conference (Optical Fiber Communications/Optical Fiber Sensors), January 25-29, 1988, New Orleans.